The present invention relates to a muting control apparatus, and in particular to a muting control apparatus which produces indication for a muting control in accordance with an error signal which indicates whether each data block of a digital audio signal has errors.
As known, a rotary head type digital audio tape recorder (hereinafter simply referred to as R-DAT) has been developed, in which an analog audio signal is digitized and a digital audio signal thus obtained is recorded on and reproduced from a magnetic tape by means of rotary heads, which are mounted on a head drum.
Such an R-DAT encodes, at one time, the predetermined number of samples obtained by sampling an analog audio signal to form a digital audio signal, and then records this digital audio signal every 128 data blocks on one track of a magnetic tape by means of two rotary heads which scan the magnetic tape alternately. As another recording method, a recording method using interleaving is employed. In the recording using interleaving, odd samples located at the odd sampling numbers and even samples at the even sampling numbers are mutually separated. Then, the digital audio signal relative to the odd samples is recorded on odd tracks by the first rotary head and the digital audio signal relative to the even samples are recorded on even tracks by the second rotary head.
Further, the R-DAT carries out an error check for each data block of the digital audio signal read out from the magnetic tape by the first and second heads. In this operation, an error signal of a high (H) level is generated for each data block having errors and the error signal of a low (L) level is generated for each data block having no error, as shown in FIG. 1 (A). In addition, one pulse called a count pulse CP is generated for each data block, as shown in FIG. 1 (B). Those signals are produced by a signal process circuit in the R-DAT.
In the reproducing operation, the data block having errors introduces noise on an analog audio signal converted from the reproduced digital audio signal. In this case, such noise on the analog audio signal is temporary when part of data blocks of the digital audio signal has errors. This means that voice or sound corresponding to this analog audio signal does not have remarkable problems in view of the sense of hearing. However, when there exist many data blocks having errors, remarkable problems in view of sense of hearing are brought about. In order to avoid these problems, the R-DAT uses a muting control for muting the audio signal having noise. In a conventional muting control apparatus, an error rate of the reproduced digital audio signal is measured for the muting control. The error rate is a ratio of the number of data blocks having errors out of given data blocks with respect to that of all the given data blocks. The error rate used for the muting control is obtained by calculating the error rate of the digital signal reproduced by the first rotary head as well as the error rate of the digital signal reproduced by the second rotary head, and then by calculating the mean value of those error rates. Then, the muting process is initiated when the mean value exceeds a predetermined value.
By the way, when blinding due to dust or the like deposited on gaps of the rotary head is occurred, this rotary head no longer reads out the digital audio signal recorded on the magnetic tape.
For example, assume that muting for the reproduced audio signals carried out when the error rate of the reproduced digital audio signal exceeds 50%. It is further assumed that the first rotary head operates generally normally with 10% of the error rate, and the second rotary head cannot reproduce the digital signal with 100% of the error rate. In this case, the error rate of the reproduced digital audio signal is equal to 55% in total, and therefore the muting control is carried out.
In the above example, consider such a situation that the digital audio signal is recorded on tracks in accordance with the recording method using interleaving. That is, the digital signal relative to odd samples is recorded on odd tracks and the digital signal relative to the even samples is recorded on even tracks. In this case, it is possible to reproduce odd samples, because the first rotary head operates normally as mentioned above. On the other hand, it is impossible to reproduce even samples, because blinding is occurred on the second rotary head. However, it is possible to obtain even samples substantially equivalent to the original even samples by interpolating the odd samples. This means that in the recording method using interleaving, the muting control is not necessary, so long as the first rotary head operates normally. However, in the above example, the conventional muting control apparatus produces indications for muting the reproduced audio signal. This is because the conventional muting control apparatus uses the mean value of two error rates with regard to two heads as the error rate for the muting control, as described before.